His design notwithstanding, the thing I actually kinda like about Luke is his personality: the guy is a complete beginner at fighting and still sucks at it, but he has a lot of passion for it and it seems like he just likes being there, not to mention that he seems to respect Ryu and the rest of the older generation. I'd say that sort of approach is certainly more endearing than the "FUCK YOU OLD MAN" schtick new-generation characters in other franchises have.
And while I understand not liking Luke, I don't really get people who are legitimately upset: we all know that he's not gonna be as popular as Capcom had hoped, which will lead to him being sidelined in the later seasons of SF6 and probably not even show up in later games (like Abel), or just being a side character (like Alex).